Sodium vapour electric discharge lamps



Feb. 24, 1959A S ,1 R, RIGDEN 2,875,358

SODIUM VAPOUR ELECTRIC DISCHARGE LAMPS Filed Jan. 25, 1955 1 /l l l I l l 2 11 6&'18178'7 3 z 1621 15 F161 United States Patent O SODIUM VAPOUR ELECTRIC DISCHARGE LAMPS Sydney Alfred Richard Rigden, London, England, assgnoil io g'IlhedGeneral Electric Company Limited, London,

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Application January 25, 1955, Serial No. 483,961

'Claims priority, application Great Britain January 27, 1954 Claims. (Cl. 313-25) gas for enabling the lamp to start and the sodium vapour discharge to be developed.

In order that such a lamp should operate at `high efiiciency, it is desirable tirstly that the current density of the `discharge should be low and secondly that the sodium vapour pressure `in normal operation should be at `or near an optimum value.` In present practice this optimum value is usually arranged to be somewhat below the pressure giving maximmneiciency in order to obtain a suitably long useful life for the larnp by reducing the 'elect of the sodium vapour attaching and blackening thefinternal surface ofthe discharge envelope and to avoid unduly rapid loss of elciency resulting from an uneven distributionof sodium metal caused by thermal migration, which etect is accelerated by the use "of high vapour pressures.

The first of the above requirements calls for the use of a discharge envelope of relatively large diameter, but as the diameter of the envelope is increased, its wall temperature for a given wattage dissipation in the discharge, and with it the operating vapour pressure of the sodium, is reduced, and too wide an enveloperesults in too low a sodium vapour pressure for high efficiency; a compromise has, therefore, to be madenbetween these requirements, and in order to obtain the best compromise it is desirable to provide means for conserving the heat of the discharge envelope to `enable as wide a diameter envelope as possible to be used consistent with its wall temperature not falling to an undesirably low value. The heat conserving means usually empolyed consists of a highly evacuated outer jacket into which envelope is fitted. j

Two main forms of such evacuated outer jacket have been used or proposed. The form at present` morst commonly used consists of a double-walled hollow cylinder closed at one end and the space between the walls of which is sealed and highly evacuated. The discharge tube, usually in the shape of a letter U, is inserted into the jacket from Vthe open end, and the lamp is operated with air present in the space between` the discharge envelope and the inner wall ofthe outer jacket, the evacuated space between the inner and outer walls of the jacket,`

which is usuallyknown as a Dewar jacket, providing the necessary heat conservation. The inner wall of the Dewar jacket serves as a screen for the absorption of l infra-red `radiation from the discharge envelope, which screen is protected by the evacuated space from the loss of heat tothe outer atmosphere and thus helps to keep up the temperature Aof `the discharge envelope; the screen infra-red radiation emitted by the latter. i

\ The other form of outer jacket referred to consists. ofI

a single-walled envelope into which the discharge en velopeis sealed, the space between the discharge envelope and the jacket being highly evacuated.` For further reducing the heat loss from `the discharge envelope in such a lamp, it has been proposed effectively to duplicate the conditions `obtained with a Dewar jacket by surrounding the discharge envelope with a `heat-conserving cylindrical glass sleeve, containing both limbs ofthe U where the envelope is of U shape, and intervening between the discharge envelope and the outer jacket; this sleeve is insulated from the outer `atmosphere by an evacuated space, asis the inner wall ofalDewar jacket, and serves to absorb and relie-ct infra-red radiation from the discharge: A

However this form of lamp has not come into widespread use since it is little, if at all, more eiicient than the Dewar jacketed lamp and the latter has` particular advantages which have rendered it preferable. Thus it is rice `less expensive in the long run, since the Dewar jacket can be made separately from the discharge envelope and the jacket and envelope simply assembled together without sealing; the Dewar jacket has alonger life than the discharge envelope and the same jacket can be re-used, usually more than once, with a different dischargeenvelope at the end of anormal lamp life. Also with the Dewar-jacketed lamp the fact that air is presentin the space immediately surrounding the discharge envelope helps toequalise the temperature distribution of the latter in operation of the lamp. A disadvantage introduced by this last feature is that the presence of moisture in this air under conditions of, high humidity, such as may be l `'experienced by a lamp used for street-lighting, tends to the discharge render star-ting of the lamp difficult or unreliable; in addition it is diicult to tell when a Dewar jacket has failed, and repeated use of such jackets can readily leadV to use of a lamp with a poor Dewar jacket, but. despitethese disadvantages the `Dewar-jacketed `lamp has beeny accepted for many years as the best form of sodiurri lamp. A combination of `the two forms of outer jacket above referred to, which combinationj may be described as a Dewar jacket having the discharge envelope sealed therein and the interiorevacuated, has also been proposed, but such a combination is very expensive tovmake and is not much more etiicient than `either the single-wall jacketed lamp or Dewar jacketed lamp alone. l 1 4 We have now found that by the use of a particular form of discharge envelope and heat-conserving sleeve arrangement in a single wall type of evacuated jacket,

increases of eliiciency can be obtained which are so surprisingly great as to render such lamps preferable `to the Dewar-jacketed lamps for at least some uses. o

According to the present invention,` a sodium vapour electric discharge lamp of the positive `column type has an elongated tubular discharge envelope of U-shape Vco`n- `taining electrodesfor the passage of anelectric discharge and a quantity of sodium for providing; a sodium vapour discharge in normal" operation of the lamp together with a iilling of rare gas for enabling the lamp to start" and the sodiumvapour discharge to be developed,`the `discharge envelope is sealed into a single-walled outer jacket with a highly evacuated space between the inner surface ofthe jacket and the outer surface of` the `discharge envelope, and `each limb of the discharge cnvelope is individually provided with a sleeveof transparent heat-insulating material which lits closely round the limb along the whole or` the greater part, of the length of the limb.

The term tits closely` implies, for the purposes of this specilicationythat the internal diameter of the sleeve is not more than 50% greaterthan the external diameter of the limb of the discharge envelope on which it is fitted,

l the diameters being taken din` a cross-section at right v Aof fthelamp is preferablyused.

The statement that the said sleeves in radiation absorption.

envases angles to the axes of the sleeve or limb. Usually the sleeves and limbs will all be of circular cross-section, but where this is not the case each said diameter is to be taken asethe maximum diameter Ain the .corresponding cross-section, that is to say, as the longest straight line which can :be drawn joining two points on the inner or v outer periphery, yas vthe case may be, of the cross-section. With the dimensions at present usual for sodium lamps of the kind .used for street light purposes, this means that -each sleeve will usually be everywhere Ynearer to the outer surface of the limb of the discharge envelope on which ,itis iitted than it is to the inner surface of the youter ljacket. Preferably the internal diameter of each sleeve is only slightly greater, rfor example not more than two millimetres greater, than the external diameter of the limb on which it is tted; the sleeve may be a sliding fit on the limb but must not be in such intimate contact with it that the sleeve constitutes in effect `(as regards thickening of the wall of the Qnot -in any case be less .than that customarily used in the yevacuated Vouter jackets of known lamps of thekind rto-which this invention relates,.that is to say, the residual gas pressure must Inot be substantially higher than 10-3 millimetres of mercury. yA vsuitably high degree of Vvacuum can beensured during manufacture of the lamp -fby vexhausting the` outer jacket to a high degree, preferably at least 4 vmillimetres mercury, and then dis persingagetter Within the jacket to clean up residual gas; the getter should not, of course, be so vdispersed as to obscure .any appreciable part of the outer jacket or dischargeenvelope'through which light is required to emerge in use'of'the lamp. Theuse of a getter is also desirable for ensuring the maintenance of a high degree of vacuum within the outer jacket, in use of the lamp, by absorbing :.occludedpgas which :may beliberated from the walls of .the Vjacket and discharge envelope, or from other parts Within Vthe jacket, and a sucient excess of getter for absorbing such occluded-gas throughout 4the normal'life a lamp in ac- Con-dance with the invention are of transparent heatinsulating material means .that the material must be a good fabsorber 'or reflector fof infrared `radiation so as,

in operation of the lamp, to reduce the rate of loss of heat `from the limbs -of the dischargeenvelope on which thesleeves are fitted; the term transparent (which term inthis'specication includes'translucent) means that the @sleeves must absorb little or vnone of the 1visible radiation from the discharge.

The wall thickness of the sleeves should, of course, Vbe such yas to .give a high infra-red In Vgeneral glass will be the most suitable and con- .-venient material ,for the sleeves,jbut other materials, for example quartz, which have similar properties and are Y otherwise suitablejfor use in lamps in accordance with the invention, andV in particular .are capable of with- -standing fthe heatdeveloped in, operation of the lamp .ing means vare not 'necessary for securing .the required A high lamp efficiencies, `and the sleeves may merely be slid onto the limbs and retained in position'without the use :of spacing means and without being coaxial with the limbs. Howeverpthe use of spacing means -mayin some .cases be desirable for.preventing'rattling'of thesleeves on v.thedischarge envelope.

discharge envelope, each sleeve may be attached to the limb on which it is fitted at one or more regions whose total axial length is small compared with the axial length of the said limb. Where a sleeve is attached to the limb of the discharge envelope on which it is tted in this way, it should be attached .to it for only a relatively short axial distance, so that the area of contact between the limb and the sleeve will be small compared with the surface area ofthe discharge envelope andthe heat conserving eect of the sleeve will not be appreciably reduced.

It such attachment of the sleeves is used, preferably each sleeve is attached to the limb at a single region only, and in such an arrangement the attachment of each sleeve to its respective limb is preferably elected at the free end of the corresponding limb of the discharge envelope. In a lamp in which the electrode leads are sealed into the ends of the envelope by means of pinches, as is usual in known types of sodium vapour lamp, and the sleeves are `of glass, each sleeve may be sealed to the pinch formed at the -end of the limb over which the sleeve is fitted, provided that the thermal expansion coefficient of the glass of the sleeve is simlar to that ofthe glass of the discharge envelope.

ln the manufacture of a lamp in'accordancewith the invention where this latter construction is employed the sealing in of the electrode lead or leads and the sealing of a sleeve to the respective limb of the discharge envelope may conveniently be carried out in a single 0peration, the lead or leads, the discharge envelope and the sleeve being held in predetermined relative positions in which they are. to1 be sealed together, lfor, example rlimb are ysqueezed together, for example by a pair ,of

metal jaws, so as to vform a pinch -in which the electrode lead(s) is/are sealed, and so that the sleeve and the limb .are sealed together .in a xed predetermined position after the cooling .ofthe glass below its softening temperature.

The sealing of the two sleeves to their respective limbs by this method may be eiectedsimultaneously if desired.

It will be appreciated that where each sleeve is attached to a limb at more than one region,l the regions of attachment `should not'hermetically close the space 'between the limb and the sleeve bounded by these regions, in order to allow'said space to be'exhausted when th outer jacket is evacuated.

With some lamps --in accordance with the invention a further increase in efficiency may sometimes be obtained by providing one or more further sleeves of transparent heat-insulating material of the Vkind aforesaid oneach limb of the discharge envelope, each such further sleeve extending-along the whole or greater part of the length of -tne corresponding limb of the discharge envelope.

' The presence of such further sleeves Vfurther reduces v the'rate of Vloss of heat `from the discharge envelope, which enhances the lamp efciency, but also reduces slightly the emission ofuseful light by absorption which detracts from the lamp efficiency, and in practicethere will in general'be little `advantage in using -more- Athan two, oratfthe most three, sleeves on each limb of-:the discharge envelope. v l l Preferably each such further sleeve tits closely round the sleeve which it immediately surrounds, that, is to say, the Yinternal diameter of each said further .sleeve 'is preferably not more than 50% greater than the external diameter of the sleeve whichV it immediately surrounds, the diameters being .taken in va cross-section atr right angles to ,the axes of Athesleeves, said diameters to be taken as the maXimllm diameters (ashereinbeforesaid) -in the :cross-section if Vthe, sleeves .are not of circular 'cross-section. 4Preferably said diameters differ by `only a few millmetres, for example not more than two millimetres. l i

The plurality of sleeves on each limb may be loosely tted, or they may be supported Vfrom each other and the discharge envelope by suitable spacers, or they may be attached together at one or more regions of small axial length, -for example with glass sleeves of suitable thermal expansion coeiicent they may all be sealed together and to the pinched end of the limb ofthe discharge envelope at the time of forming said pinch.

The elect of the sleeves in reducing the rate of loss of heat from the discharge envelope in a lamp in accordance with the invention may be utilised to increase the lamp etciency in two diterent ways; thus either the lamp may be arranged to operate at a higher sodium vapour `pressure than possible for the same current density with a lamp of similar construction not provided with the sleeve or sleeves, or the diameter ofthe discharge envelope in the lamp in accordance with the invention may be increased over that used in the unsleeved lamp to enable the former to attain the same vapour pressure at a lower current density than is possible with the latter.

In general it will be preferable to utilize the effect in the second of these two ways, since in most `cases an increasein the sodium vapour pressure is liable to shorten the useful life of the lamp by increasing the rate of attack of the sodium vapour on the inner glass surface of the discharge envelope and/or by accelerating the rate of sodium migration within the discharge envelope. In some cases, however, a combination of the two ways may be used. t

Two sodium vapour lamps in accordance with the invention will now be described by way of example with reference to Figures 1 to 4 of the accompanying drawings, in which Figures 1 andr2 represent two side views ofone of the lamps taken substantially at right angles `to each other, and Figures 3 and 4 represent similar views of the second lamp, the lampsbeing shown in part section along the lamp axis in each of the fourg- `ures; the same reference numerals are employed to denote similar parts of the two lamps in each figure of the drawings.

Thus referring to Figures 1 and Z the lamp shown therein comprises a substantially U-shaped discharge envelope 1 of glass, each limb 2 of which is approximately 32 centimetres long and has an internal diameter `of l5 millmetres, with a wall thickness of about 1.0 millimetre, the inner surface of the envelope being of sodium resistant glass. The discharge envelope is iilledA with neon containing 0.5% by volume of argon at a total pressure of 10 millmetres of mercury, together with about 1 gramme of sodium. The lamp electrodes 3,` which are of the kind `employed `in known kinds of sodium lamp, are mounted `near the ends of the discharge envelope so as to provide an arc length of `600 millmetres. i i j The discharge envelope 1 is mounted withina cylindrical glass outer jacket 4, of approximately 50millimetres `outside diameter and wall thickness of about 1.`5 millimetres, closed at one end by a dome-shaped end portion 5 and at the opposite end by a pinched foot-tube 6, the outer ends of the leads 7 to the lamp electrodes 3 being connected to two stout mounting wires 8 which are sealed `into the pinch 9 of the foot-tube forrsupporting the envelope in position within the jacket, the mounting wires connecting the leads 7 to two contact` terminals 10 of a bayonet ltype lamp cap 11 which isfsealed by means of a suitable cement to the end of the outer jacket.` The mounting wires `8 themselves pass through holes ina circu lar mica disc 17 which is heldin position coaxially within the outer jacket 4 by means of an additional support wire 18 sealed into the pinch between the'mounting wires,

the mica disc serving to prevent lateral movement of the `mounting Wires `8 and so holding the ends ofthe discharge envelope 1 in position within the outer jacket.

The curvedend of the U-shap'ed discharge envelope 1 is supportedcentrally within `the outer jacket 4 near to the domed end thereof by means of a length of Wire 12 which passes through a tubular glass bush 13 carriedbetween the limbs of the discharge envelope, a wire loop 14 which fits closely aroundthe two limbs, and which is spot welded to the wire on opposite sides of the discharge envelope, holding the envelope in position on the wire, and the two ends of the wire being spot welded to curved metal plates 15 arranged to bear against the inner surface of the outer jacket, thus supporting the envelope within the jacket.

Over each limb 2.of the discharge envelope 1 is placed a cylindrical glass sleeve 16 having an internal diameter of 18.0` millmetres, which is approximately 1.0 millimetre larger than the external diameters of the limbs, the sleeves, which have a wall `thickness of about 0.5 to 0.75 millimetre, extending the whole length ofthe limb of the discharge envelope. t

Axial movement of the sleeves 16 on the limbs of the discharge envelope is prevented by means of wire stops 8 spotwelded to the mounting wires 8, and bent so as to hold the sleeves against the wire loop 14 which surrounds the discharge envelope 1.

The space between the discharge envelope 1 and the Y outer jacket 4 is highly evacuated, and contains a getter element 19 attached to the support wire 18 for providing a coating of getter material on part of the wall of the outer jacket near to the pinched foot-tube 6, the getter material serving to absorb occuluded gas which may `be liberated from the glass walls surrounding the evacuated space during the `life of the lamp and so maintain the vacuum. The mica disc 17 besides acting asa support for the lamp serves to prevent the evaporated getter material from spreading into the spacel between the'discharge envelope and outer jacket andmayalsoserve to reduce heat losses` t-o a certain extent by reilectingtback part of the `heat radiated from the discharge envelope 1 in the direction ofthe glass pinch9. As an aid to starting the.

lamp a thin nickel wire (not shown) is connected to each of the electrode leads 7, .each wire winding helically round one of the glass sleeves 16 and being insulatingly attached to the support for the curved end of the discharge envelope 1.

For processing the lamp` partsfor the evacuation of the outer jacket, the following method was used. The jacket, with the discharge envelope mounted therein, was placed in a baking oven so` as to be shielded from direct radiation from lthe heating elements and heated conductively by hot air, and was connected to an exhaust pump provided with a liquid air trap; The outer jacket was then evacuated to a pressure ofabout 10-4 millmetres Y hausted to a pressure of` about 10*4 millimtres of mercury, and then refilled witha `second quantity of dry .oxygen-free nitrogen at the same pressure as before, the

temperatureof the lamp being maintained at 365 C. throughout this operation and for a `further 5 minutes. The jacketrwas then againconnected tothe pump and `evacuated to a` pressure oft about 10"'t millmetres of mercury,` the lamp being bakednat a temperature of 365 C. during this operation and for a further 5 minutes. Thelamp was then allowed to cool and the outer jacket sealed 01T when the lamp temperature was about C. When the lamp had iinally cooled to room temperature, the getter element was heated to cause getter material to evaporate and condense on the adjacent parts of the wall of the outer jacket. v

This lamp is intended to be operatediwithfla` nominal lone ou the wire;

wattage dissipatin of 85 -watts and a current density Vof 0.135 amp. per square eentimetre, and with such a `Ilanips'o Aoperated we 'have succeeded inobtaining an ini- -tial luminous eflic'iency Iot' about 90 lumens .per watt.

v 'Here .it may be remarked that --where in this specification 'speciiic luminous -eiciencies are referred to, these -were measured by a spherical integrating .photometer of -foot diameter' iitted with a photoelectric unit approximately corrected to the C. l. E.' average eye response; the individual measurements are subject to the usual 'limits of accuracy `of commercial photometry of about i5% relative to true values, but the diierences between Vany two eic'iency measurements have a higher degree of accuracy and indicate the differences in eliiciency with an error of not more than 2 or 3 lumens per watt, and probably less than this.

For a lamp similar .to that just described, having the same `arc length of' 60() .millimetres and arranged to operate with the same wattage dissipation as the lamp described with reference to Figures l. and 2, but not pro vided with the glass sleeves 16 surrounding the limbs of the discharge envelope 1 we found it necessary to employ a discharge envelope of smaller diameter, that is to say of about l2 millimetres bore, in order to maintain the 'same .vapour pressure in operation of the lamp. With such an 'envelope Vthe lamp operated at a current Vdensity of'0.52 amp. per square centimetre for an 85 watt dissi quarecentimetre was used to give the samevapour presf sureas before and the initial luminous etliciency was found to be about 95 lumens per watt.

lt will be appreciated that each limb of the discharge envelope in a lamp .substantially as described with ref- ,erence toFigures l and 2 could be provided, if desired, with 'a further glass sleeve litting closely over .the sleeve '16. 'In one such lamp, having a discharge envelope with -anrarc length of 400 millimetres and a tube bore of l5 millimetres, and arranged to operate at a current density o1.z approximately 0.34 amp. per square centimetre, but otherwise constructed substantially as described by way Vof example with reference to Figures l and 2 of the draw* ings with the addition, `onveach limb of the discharge envelope, of a further lsleeve having an internal diameter approximately 1 millimetre greater than the external `diameter of. the sleeves `16 and a wall thickness of between 0.5 to 0.75 millimetre, the luminous eiiiciency 1under the operating .conditions referred to was found to vbe about 100 lumens per watt.

Referring now to Figures 3 and 4, the lamp shown .therein alsoV comprises a .U-shaped discharge envelope 1 mounted within a cylindrical glass louter jacket 4, the internal diameter of the discharge envelope being about .l5 yrnillirnetres and the length -of the limbs 2 of the envelope being such as to provide an arc length of 600 millimetres between the lamp electrodes 3; the envelope, which has a wall thickness of about one millimtre, hasV the'same gas and sodium metal i'illing as the lamp `described with reference to Figures `l .and 2.v

,The curved end 23-of the discharge envelope l is supportedwithin the jacket 4 by means of a length of iwire 1 2. which passes through a tubular glass bush 13 .carriedbetween the limbs 2 of the envelope, as in the -glarnp shown in Figures l and 2, and two curved metal wire loop 14 which tits closely round the two limbs and spot welded to the wire 12 on opposite Vsides of the envelope prevents ,transverse movement of the enve- At the other end ofthe .lamp the outer jacket is closed by a pinched foot ,tube .G'having two mounting wires 8 sealed into the pinch and connected at their outer ends tothe terminals 10 of a bayonet type lamp cap 11 secured to :the jacket as vin Vthe lamp previously described. In this lamp however the ends of the mounting wires 8 lwithin ,the jacket are :bent back away from the discharge envelope 1 so as lto lie along the foot tube, and are connected to the respective electrode leads 7 by means of two nickel sleeves 20, one pair of leads 7 and the bent back end of the corresponding mounting wire 8 being inserted into .the same end of one of the sleeves and being secured in place vby spot welding. By this method of mounting, the discharge envelope can be located close to the foot tube 6, lthereby reducing the length of outer jacket necessary kfor housing the envelope.

A .sheet of mica 1 7 extends across the outer jacket with its ends in contact with the inner surface thereof aS an additional support for thedischarge envelope as in the lamp shown in Figures l and 2, although in this lamp the electrode leads 7 themselves extend through holes provided in the sheet', the connection to the mounting wires S taking place on the opposite side of the `sheet to the discharge envelope. s The space between the discharge envelope 1. and the outer vjacket 4 is exhausted and contains a getter Velement 19 welded to the end ofa support wire 1S which is sealed at its opposite end into the pinch of the foot tube 6 between `the two mounting wires 8, the end of the support wire 158 to which the getter element is attached beingpbent back in -a similar manner to the mounting wires S so as to lie along the foot tube 6.

In accordance with the invention there is iittedover each limb 2 of the discharge envelope 1 a cylindrical transparentV glass sleeve 16 having an internal diameter of approximately 18 millimetres, which is of the order of 1.0 millimetre larger 'than the external diameter of the limbs, the sleeves, which have a wall thickness of about 0.5 to 0.75 millimetre, extending substantially the whole length of the limbs and being sealed to the limbs on which they are fitted, by the sealing of one end of each sleeve to the pinch 24 at the end of the limb into which the electrode leads '7 are sealed. The sleeves 16 are thus supported ,coaxially on their respective limbs, and rattling or' the sleeves is thereby prevented. Moreover the sealing of the sleeves to the limbs prevents relative axial movement between vthe sleeves and the limbs from taking place, thereby dispensing with the need for additional stops at the ends of the sleeves.

The sealing in of the electrode leads 7 at the ends of the discharge envelope 1 and the sealing of the sleeves 16 to the limbs 2 is conveniently elected in a singleV operation, the electrodesf, the discharge envelope l and-the the glass sleeves 16 in a similar manner to thelamp ,shown in Figures l and 2, and is intended to be operated at a nominal wattage dissipation of S5 watts and a current density of `about 0.35 amp. persquare centimetre. With `such a lamp operating under these conditions we have succeeded in obtaining ,an initial luminous efficiency of about lumens per watt.

It will be appreciated that the discharge enveloper of the lamp substantially as described by way of example with reference to Figures 3 and 4 of the drawings may be provided with one or more additional transparent heat- -insulating Sleeves if desired, kand .these additional sleeves can be ttedloosely over Ythesleeves 1 6, although in Some cases they may be attached to the sleeves 16 and to the discharge envelope 1.

l claim:

1. A sodium vapour electric discharge lamp of the positive column type having an elongated tubular discharge envelope of U-shape containing electrodes for the passage of an electric discharge and a quantity of sodium for providing a sodium vapour discharge in normal operation of the lamp together with a tilling of rare gas for enabling the lamp to start and the sodium Vapour discharge to be developed, wherein the discharge envelope is sealed into a single-walled outer jacket with a highly evacuated space between the inner surface of the jacket and the outer surface of the discharge envelope, and wherein each limb of the discharge envelope is individually provided with a sleeve of transparent heat-insulating material which ts closely round the limb along at least the greater part of the length of the limb.

2. A sodium vapour electric discharge lamp according to claim 1, wherein the internal diameter or" each sleeve is not more than two millimetres greater than the external diameter of the limb on which the sleeve is :tted.

3. A sodium vapour lamp according to claim 1, including a getter material within the outer jacket for absorbing residual gas within said highly evacuated space and maintaining a high degree of vacuum within said space during the life of the lamp.

4. A sodium vapour lamp according to claim 1, wherein the said sleeves are held on the limbs of the discharge envelope by means which restrain the sleeves against relative longitudinal displacement, but without the sleeves being attached to the limbs and without the interventlon of spacing means between the sleeves and the limbs.

5. A sodium vapour electric discharge lamp according to claim 1, wherein each said sleeve is of glass.

6. A sodium vapour lamp according to claim l, wherein each sleeve is attached to the limb on which it is ntted at at least one region 'ti/hose axial length is small compared with the axial length of said limb.

7. A sodium vapour lamp according to claim 6, wherein each sleeve is attached to the limb on which it is fitted at a single region only.

8. A sodium vapour lamp according to claim 7, in which the electrode leads are sealed into the ends of the discharge envelope by means of pinches and wherein each sleeve is of a glass whose thermal expansion coedicient is similar to that of the glass of the discharge envelope and is sealed to the pinch formed at the end of the limb over which the sleeve is titted.

9. A .sodium vapour lamp according to claim 1, wherein each limb of the discharge envelope is provided with at least one further transparent heat-insulating sleeve each of which ts closely over the sleeve which it immediately surrounds and extends along the whole or the greater part of the corresponding limb of the discharge envelope.

l0. A sodium vapour lamp according to claim 9, wherein the internal diameter of each further sleeve is not more than two millimetres greater than the external diameter of the sleeve it immediately surrounds.

References Cited in the tile of this patent UNITED STATES PATENTS 2,087,745 Verburg July 20, 1937 2,120,480 Baker June 14, 1938 2,194,300 Found Mar. 19, 1940 2,228,327( Spanner Ian.` 14, 1941 2,392,305 Beese Ian. 8, 1946 

